Skybox lighting
Skyboxes are the main source of light for most maps, and as such are one of the most important parts of making a map.
Overview
Skybox lighting enters the map through every toolsskybox-textured brush, representing direct sun/moonlight and diffuse skylight. Its basic properties are controlled by three entities used in combination (four in some games):
light_environmentdefines the direction, color and intensity of the direct sunlight, and the color and intensity of the diffuse skylight.shadow_controldefines the color, direction, and attenuation distance of the dynamic shadows that are created bylight_environment. Dynamic shadows will be buggy and unconfigurable without this entity.env_sunplaces a glowing sprite in the skybox to represent the Sun's apparent position in the sky. It has no effect on lighting aside from this.env_cascade_lightcasts harsh, real-time shadows onto the map, usually copying settings from thelight_environmententity. (in all games since
)(also in 
)light_deferred_globalcasts dynamic shadows and lighting similar to env_cascade_light (only in
)(also in 
)
Explanation
To make sense of it all, just make sure you have only one of each of these entities in your map, and that the Pitch and Yaw <angles> are set the same for all three of them. (Pitch is equivalent to the Sun's angle of elevation from the ground, and Yaw is its compass bearing.)
These lighting settings—direction, color, brightness, etc.—are fairly specific to the actual image used for the 2D skybox. The List of skies article suggests some settings for official game skyboxes. The worldspawn entity defines which skybox to use. Also note that fog will look wrong if env_fog_controller's settings don't correspond to the particular skybox in use.
Note:Pitch can be overridden for light_environment and env_sun, but not shadow_control. The Pitch override also cannot be switched off and its rotation is measured counter-clockwise from the horizontal (so straight down is -90°) whereas the <angles> Pitch's rotation is clockwise (so straight down is +90°). The workaround for this is to make sure your Pitch override merely repeats the <angles> value; for example, if your shadow_control, light_environment and env_sun <angles> are "45 -60 0", set the Pitch override to "-45".